The compressors of the type considered herein, which are used in refrigeration systems and driven by a linear electric motor, comprise a generally hermetic shell, which houses a non-resonant assembly, including a crankcase.
In this known type of construction, the crankcase incorporates a cylinder, inside which is defined a compression chamber having an end generally closed by a valve plate and by a head, and an open opposite end, through which is mounted a piston reciprocating in the interior of the cylinder and defining, with the latter and with the valve plate, the compression chamber.
The piston is coupled, generally by means of a rod, to an actuating means, which carries magnets energized by the linear motor mounted to the crankcase.
The piston used in the present compressor presents a cylindrical tubular body with an open rear end and a front end closed by a top wall which carries a suction valve. The cylindrical tubular body defines a piston tubular skirt portion which is closed, close to an end edge, by the top wall (defining a head portion in the piston). In some conventional constructions, such as that illustrated in the Brazilian patent document PI 1000181-6, the piston is obtained in a single piece. The cylindrical tubular body has its length calculated as a function of the piston balance inside the cylinder, and of the sealing provided close to the inner wall of said cylinder to avoid compressed gas leakage during the compression cycle of the compressor operation.
The rod is provided internally to the piston and presents a first end affixed to the piston, in the region of the top wall thereof, and a second end affixed to the actuating means.
The linear motor drives the actuating means in a reciprocating movement and is responsible for generating the thrust necessary for displacing the piston in the interior of the compression chamber of the cylinder and, accordingly, for compressing the refrigerant fluid in the form of gas. The piston, the rod and the actuating means form a movable assembly of the compressor, to said movable assembly being coupled a resonant spring which is mounted in a manner to exert opposite axial forces on the piston, upon its reciprocating axial displacement in the interior of the compression chamber. The resonant spring operates as a guide for the axial displacement of the piston, and also actuates on the compression movable assembly, jointly with the linear motor of the compressor. The compression movable assembly and the resonant spring define the resonant assembly of the compressor.
In some constructions, the suction of the refrigerant fluid occurs through the piston. For these constructions, the top wall of the piston presents suction openings, which are selectively closed by the suction valve generally mounted to a front face of said top wall, as described and illustrated in said Brazilian patent document PI 1000181-6.
In some linear compressor constructions in which the suction is carried out through the piston, it may be necessary to mount, in the interior thereof, a noise muffler (suction muffler), for inhibiting the transmission, via gas, of different frequencies coming from the gas flow through the suction valve and from the movement of said suction valve.
Moreover, in order to reduce the variability of the natural frequency of the compressor operation, it is necessary, in determined cases, to add an extra mass to the movable assembly, known as tuning mass, in order to reduce the natural frequency of the mechanism.
Document NZ526361 (WO2004/106737) presents a constructive form for a piston in whose interior is defined a suction muffler means (as illustrated in FIGS. 30 and 31 of said document).
In this construction, part of the interior of the piston defines, directly with its inner wall, a pair of muffling chambers separated from one another by a dividing wall mounted around the rod portion which joins the piston to the actuating means. The muffling chambers are in fluid communication with each other, through windows provided in the dividing wall. The muffling chambers define a first noise muffler for some of the frequencies generated during the gas suction operation through the interior of the piston.
Besides the first noise muffler, said previous construction further presents, in the interior of the piston body, a second muffler which takes the form of a Helmholtz resonator, provided adjacent to an open end of the piston and which is constructed to attenuate the frequencies close to that generated by the compressor operation (medium frequencies). This second muffler takes the form of an insert provided in a single piece and having a closed end, mounted around a tubular axial extension of the actuating means and which surrounds the rod, and an opposite end mounted to the actuating means. Said insert is provided with an opening turned to an annular passage defined between the external wall of the insert and the internal wall of the cylindrical body of the piston. This second suction muffler defines a Helmholtz resonator which, in this previous solution, attenuates the medium frequencies.
The same construction, described and illustrated in document WO2004/106737, further comprises a third noise muffler, in the form of a tubular insert provided in a single piece and having a closed end and an end that is opened to the annular passage defined between the external wall of the second muffler and the internal wall of the cylindrical body of the piston. This third muffler attenuates the high frequencies.
Although this construction of suction muffler provided internal to the piston is defined with the purpose of providing noise attenuation in different frequencies, said construction is only efficient when each operation frequency to be attenuated is very specific. Besides, for the attenuation of frequency bands, said previous construction does not comply with a desired acoustic dampening performance.
Besides said deficiency in the acoustic performance, this previous solution is constructively complex, requiring higher precision for manufacturing the parts which compose the acoustic muffling means, as well as greater attention and a longer assembly time.
Another drawback of said solution, which can also be applied to other known compressor solutions, refers to the difficulty in avoiding or controlling the variability of the natural frequency of the compressor operation, which requires, in determined cases, the provision of adding an extra mass (tuning mass) to the movable assembly of the compressors, as an attempt'to reduce the natural frequency generated by the operation of the compressor mechanism.